You know you are getting old when candy you bought for a nickel when you were a kid is 65 cents. In comparison moving from 9 planets to "maybe 12" with potential for millions (imagine the wheelbarrow you would need to carry that many planets!) is bound to happen over a few billion years, but is not something you expect in a single lifetime - but it has.
The International Astronomical Union is working out a defintion of "planet" in order to cope with some recently discovered objects which are larger than Pluto and are orbiting the sun. Apparently, until now there has been no scientific defintion of planet. But the discovery in 2005 of 2003 UB313 (nicknamed Xena) which orbits the Sun every 557 years or so has pretty much forced the issue.
Now at their annual conference, the IAU has issued some guidelines for the definition of planet:
- A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.
- We distinguish between the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane, and other planetary objects in orbit around the Sun. All of these other objects are smaller than Mercury. We recognize that Ceres is a planet by the above scientific definition. For historical reasons, one may choose to distinguish Ceres from the classical planets by referring to it as a “dwarf planet.”
- We recognize Pluto to be a planet by the above scientific definition, as are one or more recently discovered large Trans-Neptunian Objects. In contrast to the classical planets, these objects typically have highly inclined orbits with large eccentricities and orbital periods in excess of 200 years. We designate this category of planetary objects, of which Pluto is the prototype, as a new class that we call “plutons”.
- All non-planet objects orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.
For a more scholarly take on the rules and the potential loopholes in them I encourage you to visit Phil Plait's "Bad Astronomy".
UPDATE: Things change. Pluto is no longer a planet
